The answer to this is A. 650. You find the surface area of the two cubes, then the rectangular prism, then you add them together.<span />
By first principles, the derivative is

Use the binomial theorem to expand the numerator:


where

The first term is eliminated, and the limit is

A power of
in every term of the numerator cancels with
in the denominator:

Finally, each term containing
approaches 0 as
, and the derivative is

The slope in short is:
4/7